In the manufacture of bevel and hypoid gears with curved flank lines, the cutting tools utilized are primarily face mill or face hob cutters, such types of cutting tools are well known in the art of gear manufacture. In face mill cutters the cutting blades are arranged in the cutter head such that one tooth slot is formed with each plunge of the cutter and the cutter must be withdrawn and the workpiece indexed to the next tooth slot position in order to form the next tooth slot.
Face hobbing comprises cutting blades arranged about a cutter, not in line with each other, but in groups, with usually two or three cutting blades per group. In two-blade groups, such as disclosed by U.S. Pat. No. 4,575,285 to Blakesley; U.S. Pat. No. 4,621,954 to Kitchen et al. and U.S. Pat. No. 4,525,108 to Krenzer, the blade pair comprises an inner cutting blade and an outer cutting blade. In the three-blade group, such as disclosed by U.S. Pat. No. 3,760,476 to Kotthaus, a "bottom" cutting blade is included along with an inside and outside cutting blade.
Unlike most face milling processes, in which all cutting blades pass through the tooth slot during its formation, face hobbing comprises each successive group of cutting blades passing through respective successive tooth slot with each blade in the group forming a cut completely along the longitudinal portion of the tooth slot. The cutter and the workpiece rotate in a timed relationship with each other thereby allowing continual indexing of the workpiece and continual formation of each tooth slot of the gear. If the hobbing process is of the generating type, the appropriate generating motions are superimposed with the timed relationship rotations of the tool and workpiece. Thus, in face hobbing, a single plunge of the cutting tool results in all tooth slots of the workpiece being formed.
Cutting tools for face hobbing processes usually consist of disk-shaped cutter heads with stick-type cutting blades, made from bar stock high speed steel (HSS) or carbide for example, which are inserted and positioned in slots formed in the cutter heads. Each cutting blade comprises a face portion oriented at a predetermined angle known as a side rake angle, cutting edge, a cutting side surface oriented at a predetermined relief angle, clearance edge, clearance side surface oriented at a predetermined relief angle, and a top surface.
In one type of cutting blade, such as that shown in previously disclosed U.S. Pat. No. 3,760,476, the cutting edge is inclined with respect to a plane containing the cutter axis (axial plane) that is oriented rotationally to contact the cutting edge, the angle of inclination being known as the effective hook angle. The effective hook angle (regardless of the number of blades per group) is comprised of two elements, the built-in hook angle and the cutting blade hook angle. The built-in hook angle is the angle of a blade mounting slot machined into a cutter head. This is the angular orientation of the cutting blade body when mounted in the cutter head and is usually in the range of about 4.degree. to 12.degree.. The other hook angle is the actual front face angular orientation on the cutting blade. In face hobbing, the effective hook angle, which is the angle resulting from the built-in hook angle and the actual cutting blade front face hook angle, is preferably zero degrees (0.degree.). The skilled artisan will also understand that in cutting blades having a side rake angle, the pressure angle of the cutting edge, or any change thereof, will also have influence the effective hook angle.
In one type of cutting tools (for example U.S. Pat. No. 3,760,476 discussed above) the cutting blades are sharpened by grinding the cutting side surface, the clearance side relief surface and the front face. These cutting blades will hereafter be referred to as "three-side-ground" cutting blades. By grinding the front face, adjustments to the side rake angle and the hook angle may be effected. Such changes may be utilized to keep the effective hook angle at 0.degree. or to influence tooth surface geometry. However, by grinding the front face, any coatings located on the front face are destroyed.
In another type of cutting blade, usually found in the two-blade per group cutting tool comprising an inside cutting blade and an outside cutting blade (previously discussed U.S. Pat. No. 4,575,285 for example), the cutting blades are sharpened by removing stock material from the cutting side and clearance side surfaces only (hereafter "two-side-ground" cutting blades). Thus, the front face and any coating materials (e.g. TiN, TiAIN) located on the front face are preserved during sharpening. However, in the two-side ground blade, the front face is not ground during sharpening and, therefore, there is no control of effective hook angle and less flexibility to control tooth surface geometry since the side rake angle and hook angle adjustments, obtained by grinding the front face, are not available.
It can be seen, therefore, that in three-side-ground cutting blades, any hook angle brought about by formation of the particular pressure angle and side rake angle can be adjusted by grinding the front face to form a 0.degree. effective hook angle. However, in cutting blades such as those of U.S. Pat. No. 4,575,285 for example, any effective hook angle must remain on the cutting blade. As an example, with a pressure angle range of 16.degree.-24.degree. and a side rake angle of 10.degree.-20.degree., an effective hook angle of about +/-4.degree. is formed on the above cutting blade. The +/- indicating the hook angle is positive on one of the inside or outside cutting blade and negative on the other inside or outside cutting blade.
Thus, it can be seen that one member of a gear pair cut with a three-side-ground type of cutting cannot be meshed with the other member of the gear pair cut with a two-side-ground type of cutting blade. The mating members will not mesh properly due to the flank twist, profile crowning and spiral angle differences introduced in the member cut with the two-side-ground cutting tool and having an effective hook angle of, for example, +/-4.degree..
It is an object of the present invention to provide two-side-ground cutting blades for cutting toothed articles heretofore cut by three-side-ground cutting blades.
It is a further object of the present invention to provide two-side-ground cutting blades whereby an effective hook angle modification is introduced such that toothed articles cut by the two-side-ground cutting blades are essentially identical to those articles heretofore cut by the three-side-ground cutting blades.
It is a further object of the present invention to provide a cutting tool comprising the above two-side-ground cutting blades.